Nuclear reactors such as pressurized-water nuclear reactors, comprise a pressure vessel in which is arranged the core of the reactor consisting of fuel assemblies and a primary circuit comprising at least one loop on which is arranged a steam generator, inside which the primary coolant fluid of the reactor heats up and vaporizes the feedwater. Each of the loops of the primary circuit comprises very thick large-diameter pipes in which the primary coolant fluid circulates. One of the pipes, or the hot leg, connects the pressure vessel to the steam generator and ensures the transfer of the primary fluid, heated up in contact with the core, into the primary part of the steam generator.
Another pipe, or the cold leg, ensures the return of the coolant fluid into the pressure vessel after having passed through the steam generator.
In order to control and monitor the operation of nuclear reactors, it is necessary to measure the temperature of the primary fluid, which temperature must of necessity be maintained within a predetermined range in order to ensure satisfactory operation of the nuclear reactor.
One of the most widely used methods consists in sampling coolant fluid from various legs of the primary circuit, in mixing the samples obtained and in measuring the temperature of the samples of fluid after they have been mixed. These temperature measurements are carried out on the sampling lines or in headers situated at points remote from the primary pipes.
The sampling circuit comprises small-diameter pipes, the total length of which can be of the order of 60 m. Furthermore, the sampling circuit is complex and comprises numerous gates and flaps in the region of which leaks can occur, resulting in the environment of the nuclear reactor being contaminated. Decontamination procedures must then be implemented which may prove to be lengthy and costly.
Furthermore, the maintenance operatives who work on the sampling circuit may be subjected to a high level of radiation.
Devices are also known for measuring the temperature of a coolant fluid inside the primary pipes which comprise a probe, the support body of which traverses the wall of the primary pipe. These devices, termed scoops, can comprise an element in the shape of a glove finger penetrating into the primary duct and inside which the temperature-measuring probe is placed. The portion of the glove finger inside the primary pipe is perforated so that the coolant fluid can circulate in contact with the probe.
The temperature of the coolant fluid of a nuclear reactor is not homogeneous in the various sections of the primary pipes and, in particular, in the hot legs where stratification of this coolant fluid takes place.
Methods for measuring the temperature of the coolant fluid must take account of these phenomena in order for the measurements obtained to be genuinely representative and to enable efficient monitoring and control of the nuclear reactor to take place.